1. Field of the Invention
The present invention relates to an ink set, an ink cartridge, an ink jet recording method, and an ink jet recording apparatus.
2. Description of the Related Art
In recent years, the image quality of a recorded article obtained by an ink jet recording method has been remarkably enhanced, and a recorded article has increasingly been stored for a long period of time in exhibition or the like. For this reason, ink has been required to be able to form an image superior not only in color developability but also in image storage stability. The image storage stability includes various characteristics such as gas resistance, light resistance, humidity resistance and water resistance, but it is necessary to particularly enhance the gas resistance and the light resistance of the image storage stability, supposing exhibition of the recorded article.
A coloring material to be used in an ink for an ink jet includes dye and pigment. An ink containing dye shows adequate color developability compared to an ink containing pigment, and can form an image having superior gloss using a recording medium having gloss. However, the image obtained by the ink containing dye is inferior in the performance of the gas resistance and the light resistance compared to the ink containing pigment.
Conventionally, a study for improving the molecular structure of the dye has been in a mainstream, as a measure for enhancing the gas resistance and the light resistance of the image formed by the ink containing the dye. However, it is hard to say that sufficient image storage stability can be obtained by enhancing the gas resistance and the light resistance of an image obtained only for an ink of a certain single color, through a contrivance of a molecular structure. This is because when the performances of the light resistance and the gas resistance are different among images formed by a plurality of inks, an image region formed by an ink having a low performance early fades or discolors than an image region formed by other inks, resulting in an insufficiently balanced image as a whole.
There are proposals concerning a method of equalizing the deterioration rate of each ink, as a method of enhancing the balance of the image storage stability for the whole image (Japanese Patent Application Laid-Open Nos. 2004-107633 and 2004-107638). Japanese Patent Application Laid-Open Nos. 2004-107633 and 2004-107638 disclose a method of enhancing ozone resistance or light-resistance of a single color in each ink constituting an ink set and a method of equalizing the performance of the light resistance or the ozone resistance among images formed by each ink. Japanese Patent Application Laid-Open No. 2008-081693 describes improving performance of an ink itself by the contrivance of a coloring material to be used in the ink, achieving the image storage stability balanced among a plurality of inks, and achieving improvement in both ozone resistance and fluorescent lamp resistance. There are also proposals concerning the enhancement of the fastness of an image by enhancing association properties of dye (Japanese Patent Application Laid-Open No. 2005-162823), and imparting fastness comparable to that of other inks to a yellow dye having relatively inferior fastness by using a dye having a specific structure (Japanese Patent Application Laid-Open No. 2007-138124).
The present inventors conducted an ozone resistance test and a xenon light resistance test in order to evaluate the image storage stability of recorded articles which had been produced by using an ink set described in each of the above-described patent documents when exhibited at places of various conditions.
As a result, it was found that the ink set in which the balance was considered from the viewpoint of the light resistance as described in Japanese Patent Application Laid-Open No. 2004-107633 and the ink set described in Japanese Patent Application Laid-Open No. 2004-107638, in which the balance was considered from the viewpoint of the ozone resistance were certainly enhanced to some extent in the point of the balance in each test. However, it was found that each ink described in these patent documents originally had a low performance of the ink itself, accordingly the image could not be considered to have sufficient image storage stability compared to the image before the test, and besides, could not achieve both ozone resistance and xenon light resistance. In addition, when the fastness of the image against light while the recorded article is exhibited in various environments is considered, the performance of the light resistance needs to be determined by the performance against the xenon light having wavelengths similar to those of sunlight. However, the ink set described in Japanese Patent Application Laid-Open No. 2008-081693, in which the balance of the fluorescent lamp resistance was considered, was found not to have a sufficient performance on the xenon light resistance. Furthermore, the ink sets described in Japanese Patent Application Laid-Open Nos. 2005-162823 and 2007-138124 were found to have approximately equal fastness among three types of inks to achieve both the ozone resistance and the xenon light resistance, but not to satisfy the image storage stability of a high level which is required in recent years.
Accordingly, an object of the present invention is to provide an ink set in which each ink constituting the ink set achieves ozone resistance and xenon light resistance at a high level, and shows a superior balance between the ozone resistance and the xenon light resistance as well. In addition, another object of the present invention is to provide an ink cartridge, an ink jet recording method and an ink jet recording apparatus, which use the above-described ink set.